Here, the expression “piece of equipment carried onboard” is to be understood as meaning any piece of equipment at least partially secured to at least one satellite. It may in particular be an observation instrument intended to carry out a space observation mission and consisting for example of one or more space telescopes, possibly spread over several satellites, or at least one radar antenna, or alternatively a plasma jet, or even a sensor.
Also, the expression “space observation mission” must be understood here to mean both missions intended to observe the Earth from space and missions intended to observe part of the universe from space.
In order to carry out observation missions, certain satellites comprise at least part of an observation instrument, for example such as a space telescope, for example of the Cassegrain, Gregory, Korsch, Ritchey-Chrétien or Newton type.
Such telescopes comprise at least one support device intended to keep a first element, such as a mirror (possibly of the primary type (often known as a collector)) a chosen distance away from a second element, such as a mirror (possibly of the secondary type) or part of a detector, installed at the focal plane where the images are formed. Such a device, together with the elements that it supports, constitutes a structure of set geometric configuration. This structure is sized in such a way that it can withstand the mechanical stresses due to gravity and the loads (or forces) experienced during the various phases of the launch, especially as the rocket in which the satellite fitted with the device is carried lifts off.
Once the mission has begun, the loads that the structure has to withstand are very low, if not practically zero. The structure therefore proves to be over-engineered throughout its operational life, leading to an inertia, particularly a transverse inertia, that is higher than is truly necessary. Since this “over-inertia” cannot be reduced in orbit, it limits the rate of change of aim point of the telescope and therefore the time during which it can acquire images, something which is penalizing particularly when it is carried onboard a so-called “agile” satellite and/or when a “mosaic image” needs to be compiled (because the field of the instrument is smaller than the field that needs to be observed).
Furthermore, the over-engineering of the support structure results in additional mass and additional bulk which mean that higher capacity rockets need to be used to carry it and/or which may limit the number of satellites that can be launched from the same rocket.